1. Field of the Invention
The present invention relates to a magnetoresistance effect element for use in a magnetic head and the like.
2. Description of the Related Art
Generally, to read out information recorded in a magnetic recording medium, a reading magnetic head with a coil is moved relative to the recording medium, and a voltage induced in the coil by electromagnetic induction generated upon the movement is detected. A method using a magnetoresistance effect head to read out information is also known [IEEE MAG-7, 150 (1971)]. This magnetoresistance effect head makes use of a phenomenon in which the electrical resistance of a certain kind of a ferromagnetic substance changes according to the intensity of an external magnetic field, and is known as a high-sensitivity head for a magnetic recording medium. Recently, as magnetic recording media have been decreased in size and increased in capacity, the relative velocity between a reading magnetic head and a magnetic recording medium during information reading has decreased. Therefore, development of a magnetoresistance effect head capable of extracting a high output even at a low relative velocity has been desired increasingly.
Conventionally, an NiFe alloy (to be abbreviated as a permalloy hereinafter) has been used in a portion (to be referred to as an MR element hereinafter) of a magnetoresistance effect head in which the resistance changes in response to an external magnetic field. The permalloy, even one with good soft magnetic characteristics, has a maximum rate of change in magnetic resistance of about 3%, and this value is too low to use the permalloy as the MR element for a small-size, large-capacity magnetic recording medium. For this reason, a demand has arisen for an MR element material with a highly sensitive magnetic resistance change.
In recent years, it has been confirmed that a multilayered film formed by alternately stacking ferromagnetic metal films and nonmagnetic metal films, such as Fe/Cr or Co/Cu, under certain conditions, i.e., a so-called artificial lattice film gives rise to a very large change in magnetic resistance by using antiferromagnetic coupling between adjacent ferromagnetic films, and a film which exhibits a maximum rate of change in magnetic resistance exceeding 100% has been reported [Phys. Rev. Lett., Vol. 61, 2472 (1988)] [Phys, Rev. Lett., Vol. 64, 2304 (1990)].
Another type of a structure has also been reported, in which although ferromagnetic films do not experience antiferromagnetic coupling, an exchange bias is applied to one of two ferromagnetic films sandwiching a nonmagnetic film by using some means other than antiferromagnetic coupling between adjacent ferromagnetic films, thereby locking the magnetization of the film, while the magnetization of the other ferromagnetic film is reversed by an external magnetic field. This forms a state in which the two ferromagnetic films are antiparallel to each other on both the sides of the nonmagnetic film, realizing a large change in magnetic resistance. This type is herein termed a spin valve structure [Phys. Rev. B., Vol. 45806 (1992)] [J. Appl., Phys., Vol. 69, 4774 (1991)].
In either of the artificial lattice film or the spin valve structure, the resistance change characteristics and the magnetic characteristics of the multilayered film change largely in accordance with the type of the ferromagnetic film. For example, in a spin valve structure using Co, such as Co/Cu/Co/FeMn, a high resistance change rate of 8% results, but the coercive force is as high as approximately 20 Oe, i.e., no good soft magnetic characteristics can be obtained. In contrast, in a spin valve structure using the permalloy, such as NiFe/Cu/NiFe/FeMn, although a good value of 1 Oe or less has been reported as the coercive force, the resistance change rate is not so high, about 4% [J. Al. Phys., Vol. 69, 4774 (1991)]. That is, the soft magnetic characteristics of the stacked film are good, but its resistance change rate decreases. Therefore, neither a constituent element nor a film structure of a stacked film which satisfies both the soft magnetic characteristics and the resistance change rate has been reported yet.
In addition, the above two types of the films have the following problems.
The artificial lattice film has a higher resistance change rate .DELTA.R/R (ignoring a magnetic field range) than that of the spin valve structure. However, a saturation magnetic field Hs of the artificial lattice film is large because antiferromagnetic coupling is strong, so the film suffers poor soft magnetic characteristics. In addition, since this RKKY-like antiferromagnetic coupling is sensitive to an interface structure, stable film formation is difficult to perform, and deterioration with time readily takes place.
A film with the spin valve structure can achieve good soft magnetic characteristics when an NiFe film is used as the ferromagnetic film. Since, however, the number of interfaces between the ferromagnetic films and the nonmagnetic film is two, the .DELTA.R/R is lower than that of the artificial lattice film. Even if a stacked film is constituted by ferromagnetlc, nonmagnetic, and antiferromagnetic films in order to increase the number of interfaces, since the antiferromagnetic film with a high resistance is present in this stacked film, spin-dependent scattering is suppressed. Therefore, no increase in the .DELTA.R/R can be expected.
In addition, when a signal magnetic field is applied in the direction of the axis of hard magnetization of ferromagnetic films suitable for a magnetic head, the magnetization of only one of the ferromagnetic films is rotated. As shown in FIG. 1, therefore, the angle defined between the magnetization of a ferromagnetic film 2 on an antiferromagnetic film 1 and the magnetization of a ferromagnetic film 4 on a nonmagnetic film 3 can be changed to only about 90.degree. by the application of the signal magnetic field. Note that a change in the angle of up to 180.degree. occurs in the direction of the axis of easy magnetization. Consequently, the .DELTA.R/R decreases to about half that in the axis of easy magnetization. Assume, for example, that the exchange bias magnetic field of the ferromagnetic film 2 on the antiferromagnetic film 1 is weakened by some method to make it possible to use the magnetization rotations of both the ferromagnetic films 2 and 4. In this case, if the film thickness of the nonmagnetic film 3 is decreased to increase the resistance change rate, ferromagnetic coupling acts between the two ferromagnetic films. Therefore, the magnetizations between the two ferromagnetic films point in the same direction when the signal magnetic field is 0. Consequently, even if magnetizations rotate upon application of the signal magnetic field, only a slight change results in the angle between the magnetizations of the two ferromagnetic films, and so the resistance change is also subtle.
Furthermore, the ferromagnetic coupling acting between the two ferromagnetic films when the film thickness of the nonmagnetic film is decreased causes deterioration in permeability. The NiFe film having good soft magnetic characteristics has a normal anisotropic magnetoresistance effect. However, in a system in which a sense current is flowed in a direction perpendicular to a signal magnetic field, when the signal magnetic field is 0 and the magnetizations of two ferromagnetic films point in the same direction, the anisotropic magnetoresistance effect obtained by the signal magnetic field and the resistance change obtained by spin-dependent scattering cancel each other out, as shown in FIG. 2.
Common problems of the artificial lattice film and the spin valve structure will be described below. First, in order to obtain a high sensitivity in a magnetic head, a current to be supplied must be increased as large as possible. If the current is increased in either of the film structures, however, the magnetization directions of some ferromagnetic films are disturbed by a magnetic field produced by this current, preventing a highly sensitive resistance change with respect to the magnetic field. More specifically, the magnetization readily points in the direction of the current magnetic field in the vicinities of the uppermost and lowermost layers of the stacked film, so the current magnetic field is strong in these portions.
Second, there are serious problems, such as the Barkhausen noise suppression and operating point bias, to be solved in applying the film to a magnetic head.
As described above, no existing magnetoresistance effect elements with the artificial lattice film or the spin valve structure using spin-dependent scattering can exhibit both good soft magnetic characteristics and a high resistance change rate .DELTA.R/R, which are essential to obtain a high sensitivity, even upon supply of a large current.